1. Field of the Invention
This invention relates to methods of manufacturing reflective articles comprising a reflective metal coating deposited on a transparent substrate.
A reflective metal coating may be deposited pattern-wise to form a decorative article, but the invention has particular reference to glass substrates bearing a continuous reflective coating. The coating may be applied to a substrate of any form, for example to an artistic object, to achieve some desired decorative effect, but it is envisaged that the invention will find greatest use when the coating is applied to a sheet glass substrate. The coating may be fully reflective, thus forming a mirror-coating.
Reflective metal coatings, e.g. of silver, are apt to be attacked by atmospheric pollution with the result that the silver layer becomes tarnished so that the required optical properties of that layer are lost. It is accordingly known to apply protective layers to such a silver layer, the nature of the protective layer being determined by the required properties of the coated substrate and by cost.
2. Description of the Related Art
Conventionally, silver mirrors are produced as follows. The glass is first of all polished and then sensitised, typically using an aqueous solution of SnCl.sub.2. After rinsing, the surface of the glass is usually activated by means of an ammoniacal silver nitrate treatment. The silvering solution is then applied in order to form an opaque coating of silver. This silver coating is then covered with a protective layer of copper and then one or more coats of paint in order to produce the finished mirror.
The purpose of the copper layer is to retard tarnishing of the silver layer, and the copper layer is itself protected from abrasion and corrosion by the layer of paint.
Of the various paint formulations which could be used for protecting a mirror, those which afford the best protection against corrosion of the copper layer contain lead pigments. Unfortunately lead pigments are toxic and their use is being increasingly discouraged for reasons of environmental health.
It has also been proposed to protect the silver coating by treatment with an acidified aqueous solution of Sn (II) salt (see British patent application GB 2252568). According to this proposal, the reflective layer of silver has a surface stratum having a population of tin atoms which is augmented as compared with the population of tin atoms (if any) in an underlying subsurface stratum by at least one tin atom per hundred metal atoms thereby conferring on such metal coating an enhanced resistance to corrosion.
According to another recent proposal, (see British patent application GB 2254339), the reflective layer of silver has been treated with a solution containing ions of at least one of the group consisting of Al (III), Cr (II), V (II or III), Ti (II or III), Fe (II) and In (I or II).
One important application of the protection treatments according to GB 2252568 and GB 2254339 is the formation of silver mirrors which do not include a conventional protective layer of copper. Such copper-free mirrors can be protected with lead-free paints.
Paints are usually applied in liquid form, for example by brushing onto the surface to be protected or by conveying the article through a liquid curtain. It is sometimes difficult to obtain a uniform coating when using this method, resulting either in the use of excess paint or in areas where the paint layer is of insufficient thickness to provide the required protection. Furthermore, liquid paints may contain volatile solvents which are hazardous to use and whose discharge into the atmosphere is generally to be avoided for environmental reasons. When paints are applied in liquid form adhesion to the underlying surface is not always as good as desired. Further, liquid paints must generally be applied in two steps, since two layers are necessary to obtain sufficient protection.